1. Field of the Invention
The present invention relates to a printer with a plurality of ink ribbon cassettes. More particularly, the invention relates to a printer which allows a print head to be opposed to any one of the ink ribbon cassettes for respective colors so that it can print characters or patterns in a desired color.
2. Description of the Related Art
A printer which is capable of printing characters or patterns in several colors normally mounts a plurality of printing ink ribbon cassettes for respective colors on a vertically movable cassette table for movement up and down the cassette table with a cam mechanism driven by a motor in order that a desired ink ribbon cassette is allowed to be opposed to a print head. This type of printer has been disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 61-182961, laid open on Aug. 15, 1986.
Another printer which is capable of printing characters is operated to move an ink ribbon up and down providing several colors with a cam mechanism driven by a stepping motor in order that a ribbon portion of a desired color is opposed to a print head. This type of printer has been disclosed in the Japanese Patent Application Laying Open (KOKAI) No. 59-194884, laid open on Nov. 5, 1984.
As shown in FIG. 1, those printers are designed to control the drive motor of the ink ribbon cassettes or of the ink ribbon in a manner to keep a rotational speed of the motor constant irrespective of load variation. When the load reaches a maximum peak, therefore, the motor rotates too fast to reach the drive torque which allows positive drive. In particular, if the drive motor employs a stepping motor, it may be stepped out.
In order to prevent the motor from being stepped out when drive torque is short, the rotational speed of the stepping motor should be set low. However, such setting results in enlarging the drive torque, which may heat the stepping motor. In particular, if a carriage is iteratively moved up and down for frequently switching the colors, abnormal heating may occur in the stepping motor and be finally burnt out. Moreover, as the rotational speed of the motor is set lower, the cassette table is slowly moved up and down, resulting in a shortcoming that it takes a long time to switch the printing ink cassette. Accordingly, it has been impractical to set the rotational speed of the motor low if there has existed a requirement for the quick and smooth switching of the ink cassette.